Vertical take-off and landing (VTOL) aircrafts are well known. Some types of such aircrafts take-off and land in “helicopter mode” with their propeller axes oriented substantially vertically. During transfer to cruise flight mode, the propeller axes are gradually rotated into a substantially horizontal position. VTOL aircrafts with fixed-diameter rotors suffer from the disadvantage of limited speed in cruise flight mode and big centrifugal forces that act on the rotor blades.
U.S. Pat. Nos. 5,642,982 and 5,735,670 disclose VTOL aircrafts with variable-diameter rotors. The latter patent describes an aircraft with a variable-diameter rotor comprising a hub having rotor blades mounted thereon wherein each of the rotor blades comprises a radially internal blade part, which is radially stationary, and a radially external blade part, which is radially movably mounted on said radially internal blade part. In each of the rotor blades, a jackscrew mechanism is provided for moving the radially external blade part with respect to the radially internal blade part. The diameter change mechanism includes, in particular, an input bevel gear, which drives one output bevel gear per blade. The output bevel gears are coupled to the jackscrews (in this case ballscrews) of the respective rotor blades. As the jackscrew is rotated about its longitudinal axis, the nuts are arranged in the radially external blade parts and with them the blade parts themselves are caused to move radially inwardly or outwardly, depending on the direction of rotation of the jackscrew.
During the operation of the rotor, big centrifugal forces act on the rotor blades. Accordingly, to move the external parts of the rotor blades inwardly, high loads have to be overcome. Additionally, with increasing rotor speed, friction between the jackscrews and the corresponding nuts increases. The high loads the jackscrew mechanism has to overcome are potentially prejudicial to aircraft safety.
To reduce this problem, a variable-diameter rotor has been proposed, in which the external parts of the telescopic rotor blades are radially movable by means of a jackscrew mechanism and wherein each blade is equipped with an additional mechanism for compensating, at least partially, the centrifugal forces acting on the jackscrew mechanism. This additional mechanism comprises a hydraulic cylinder located in each blade, a pneumo-hydraulic accumulator located in the hub and a pulley-cable arrangement. The pneumo-hydraulic accumulator is divided by a movable piston into a first volume containing hydraulic fluid and a second volume containing pressurized gas. The volume with hydraulic fluid is fluidly connected to the hydraulic cylinder. The hydraulic cylinder is fixed on a spar of the radially internal blade part between a first pulley block supported by the spar and a second pulley block supported by the piston of the hydraulic cylinder. A cable, having its first end fixed to the radially external blade part and its second end fixed to the radially internal blade part, is guided over the pulley blocks in such a way that as the external blade part moves outwardly, the cable urges the pulley blocks closer together and thereby moves the piston of the hydraulic cylinder. In turn, the hydraulic fluid flows into the pneumo-hydraulic accumulator, in which the pressurized gas is compressed further and stores the mechanical work of the motion of the external blade part.
A problem with this variable-diameter rotor is that the mechanism for compensating centrifugal forces increases the weight of the rotor blades, which is not desired. Furthermore, integration of such mechanisms into the blades is difficult due to design constraints.